This invention relates to imaging systems, and more particularly, to improved developer systems and techniques.
The formation and development of images on the surface of photoconductive materials by electrostatic means is well known. The basic electrostatographic process, as taught by C. F. Carlson in U.S. Pat. No. 2,297,691 involves placing a uniform electrostatic charge on a photoconductive insulating layer, exposing the layer to a light-and-shadow image to dissipate the charge on the areas of the layer exposed to the light and developing the resulting charge pattern by depositing on the layer a finely-divided marking material referred to in the art as "toner". The toner will normally be attracted to those areas of the layer which retain a charge, thereby forming a toner image corresponding to the charge pattern. This powder image may then be transferred to a support surface such as paper. The transferred image may subsequently be permanently affixed to a support surface as by heat.
Instead of charge pattern formation by uniformly charging the photoconductive layer and then exposing the layer to a light-and-shadow image, one may form the charge pattern by directly charging the layer in image configuration. The powder image may be fixed to the photoconductive layer if elimination of the powder image transfer step is desired. Other suitable fixing means such as solvent or overcoating treatment may be substituted for the foregoing heat fixing step.
Similar methods are known for applying the marking particles to the electrostatic latent image to be developed. Included within this group are the "cascade" development technique disclosed by E. N. Wise in U.S. Pat. No. 2,618,552, the "powder cloud" technique disclosed by C. F. Carlson in U.S. Pat. No. 2,221,776 and the "magnetic brush" process disclosed, for example, in U.S. Pat. No. 2,874,063.
Development of a charge pattern may also be achieved with liquid rather than dry developer materials. In conventional liquid development, more commonly referred to as electrophoretic development, an insulating liquid vehicle having finely divided solid material dispersed therein contacts the imaging surface in both charged and uncharged areas. Under the influence of the electric field associated with the charged image pattern the suspended particles migrate toward the charged portions of the imaging surface separating out of the insulating liquid. This electrophoretic migration of charged particles results in the deposition of the charged particles on the imaging surface in image configuration.
An additional development technique is that referred to as "wetting development" described in U.S Pat. No. 3,285,741. In this technique an aqueous developer uniformly contacts the entire imaging surface and due to the selected wetting and electrical properties of the developer substantially only the charged areas of the imaging surface are wetted by the developer. The developer should be relatively conductive having a resistivity generally from about 10.sup.6 to 10.sup.10 ohm cm and having wetting properties such that the wetting angle measured when placed on the photoconductor surface is smaller than 90.degree. at the charged areas and greater than 90.degree. in the uncharged areas.
A significant advance in developing charge patterns in a liquid development process is disclosed by R. W. Gundlach in U.S. Pat. No. 3,084,043. In this method hereinafter referred to as polar liquid development, a charge pattern is developed or made visible by presenting to the image surface a liquid developer on the surface of a developer dispensing member having a plurality of raised portions or "lands" defining a substantially regular patterned surface and a plurality of portions depressed below the raised portions or "valleys". The depressed portions of the developer dispensing member contain a layer of conductive liquid developer which is maintained out of contact with the electrostatographic imaging surface.
Development is achieved by moving the developer dispensing member loaded with liquid developer in the depressed portions into developing configuration with the imaging surface. The liquid developer is believed to be attracted from the depressed portions of the applicator surface in the charged or image areas only. The developer liquid may be pigmented or dyed.
The development system disclosed in U.S. Pat. No. 3,084,043 differs from electrophoretic development systems where a substantial contact between the liquid developer and both the charged and uncharged areas of an image bearing surface occurs. Unlike electrophoretic development systems, substantial contact between the polar liquid and the areas of the electrostatic latent image bearing surface not to be developed is prevented in the polar liquid development technique. Reduced contact between a liquid developer and the non-imaging areas of the surface to be developed is desirable because the formation of background deposits is thereby inhibited. Another characteristic which distinguishes the polar liquid development from electrophoretic development is the fact that the liquid phase of a polar developer actually takes part in the development of a surface. The liquid phase in electrophoretic developers functions only as a carrier medium for developer particles.
In the development system disclosed in U.S. Pat. No. 3,084,043 an applicator roll is utilized to present liquid developer to the surface of the member carrying the charge pattern. The amount of liquid on the applicator roll is carefully controlled by using a doctoring or metering blade. It has been found with this system that the quality C. the final images produced may depend C. on the temperature of the liquid developer in the system. It has been observed that with many liquid developers variation in temperature causes a viscosity change which effects the final developed image density. In general at higher operating temperatures the reproduced images are more dense than at lower operating temperatures. This result can be explained when it is realized that at higher operating temperatures the liquid often becomes less viscous and thus more readily transferable at the image developing station. Accordingly, to provide uniform results at an operating temperature of 10.degree. and 40.degree. C where the viscosity change can be as much as ten-fold requires a system for temperature compensation.
Prior art processes have attempted to solve this problem by using viscosity controlling additives, but such additives have not proved to be totally successful. Moreover, the difficulties associated with changes in temperature are not limited solely to the changing viscosity of the liquid developer. For example, where a resilient thermoplastic doctoring blade is used to meter or control the loading of the liquid developer on the applicating member, the changes in temperature may effect the viscoelastic properties of the blade material which, in turn, may result in different doctoring depths. Therefore, even where the viscosity of the liquid developer is being controlled, changes in ambient temperature may still result in inconsistent and poor image quality.
An apparatus and process for temperature compensation in liquid developing compositions have been provided by S. C. P. Hwa in U.S. Ser. No. 552,234 filed Feb. 24, 1975 and assigned to the instant assignee. Hwa describes an apparatus for doctoring having means for automatically adjusting the pressure of the doctoring means against an applicator in response to changes in ambient temperature. Exemplary of such adjusting means are thermo-mechanical devices, thermo-electromic devices and thermo-fluidic devices.